Reverse Osmosis (RO), nanofiltration, ultrafiltration, electrodialysis, and electrodeionization are just a few of the technologies that utilize membranes to purify water. One of the key drivers behind the economics of water purification is the amount of energy required to perform the separation. Decreasing the amount of energy required to perform the separation usually decreases the cost of water purification. As a result, there is a market need to decrease the amount or energy required to purify water.
Reverse osmosis, nanofiltration, and ultrafiltration are processes that use an applied pressure to remove impurities from water. In comparison, electrodialysis and electrodeionization processes use an electric field to remove impurities from water. A schematic of one type of standard electrodialysis (ED) process in shown in FIG. 1. A potential or voltage 110 is applied to two metal electrodes denoted as anode (+) 108 and cathode (−) 106 in FIG. 1. Stacks of alternating cation permeable membranes 104 and anion permeable membranes 102 are in between the electrodes. These membranes are typically made of ion exchange resins that only permit the passage of canons (cation permeable) or anions (anion permeable). An example of how the process works is shown in FIG. 2.
Referring to FIG. 2, in this example, water with a high salt content (e.g., NaCl) flows into the top of the assembly. As the water flows to the bottom, Na+ ions 212 and Cl− ions 210 are removed from the water. The purified water 214 flows out the bottom. Cations (Na+) 212 and anions (Cl−) 210 are concentrated in one compartment (brine) 216, while product water is produced in the other compartment. A cathode 206 and an anode 208 are positioned on either side of an alternating series of cation selective permeable membranes 202 and anion selective permeable membranes 204. Cations and anions must diffuse from the input stream, through the membranes, and into the brine or product streams. This diffusion process is driven by the applied potential or voltage (V). The amount of cations or anions removed in a given amount of time relates to the current (I). The electrical resistance of the membrane can be defined as R. Using Ohm's Law, V/R−I. This implies that for a given applied voltage, the greater the electrical resistance of the membrane, the lower the current or amount of ions removed from the water per given time. These relationships imply that lowering the resistance of the membranes should increase the current or amount of ions removed per given time.
The electrodeionization (EDI) process very similar to the ED process. In this case, ion exchange resin beads are used to fill in the spaces between the membranes in FIGS. 1 and 2. The ion exchange resin beads assist in removing small traces of cations and anions that are present in the feed water. Cations and anions are still transported through the membranes and the Ohm's Law discussion described above is still valid.
In order for membranes to function properly in ED and EDI applications, they must be selective for either cations or anions. In other words, anion permeable membranes should enable only the transport of anions, while cation membranes should enable only the transport of cations. As a result, traditional ED and EDI membranes are nonporous meaning that there are no pores large enough to allow bulk flow of water and ions. Ion transport through these nonporous membranes tends to be slow. This is one of the reasons for the high electrical resistivity of traditional ED and EDI membranes.
ED and EDI membranes are commonly made from polymers that have poor mechanical properties. The types of polymers that transport ions also tend to have poor mechanical properties. The thickness of the membranes must be great enough to withstand factors such as packing stacks of the membranes and withstanding pressure differentials across the membranes.
Commercial ED and EDI membranes not only suffer from high electrical resistances, but also must be stored in special solutions (i.e., stored wet). The performance of these membranes decreases when they dry out. These membranes are continuous, i.e., they do not have pores and are relatively thick. These features slow transport of ions across the membrane.
Prior art sources have focused on using advances in the chemistry of ion exchange resins to enhance the transport properties of membranes used in ED and EDI processes. Various functional groups have been added to the polymer chains of the ion exchange resins used to fabricate the membranes. These functional groups are reported to enhance the ability of the membrane to transport only cations, only anions, and/or only certain rations or anions. These types of improvements either alter the chemistry of the polymer backbone or alter side chains on the polymer. Also, asymmetric or composite membranes can he prepared. This approach, which has traditionally been used for gas separation membranes, was applied to ED and EDI membranes. It results in a thin dense layer (nonporous layer) on the top of a microporous hacking. The procedure to form these asymmetric structures is complicated and requires casting from solvents and working with emulsions. It has been shown previously that residual solvent in asymmetric films influences the transport properties of the films. Transport of ions still relies on the same mechanisms described previously.
Charge-mosaic membranes and bipolar membranes attempt to increase the efficiency of separation processes by combining cation and anion selective membranes into a layered structure. These membranes are still based on traditional on exchange resins and the transport of ions is very similar to previously described art. They are nonporous as are the layers containing functional groups on the polymers that drive the separation process.
Nanoporous structures have been discussed in the prior art for separation of charged macromolecules, e.g., DNA. These systems employ the use of ion-track etched polymers that have been coated with metals. A separate charge must be applied to the metalized polymer in order to create a surface charge. This system requires special asymmetric pores. In addition, a voltage is not only applied to the metalized polymer, it is also applied across the metalized membrane. This creates a very energy intensive and costly system due to the need to apply multiple voltages and the need to design specific asymmetric pores.
In order to reduce the energy required to purify water it would he desirable to decrease the thickness of the membrane. In addition, it would be desirable to transport the ions through the film using a mechanism different than the mechanism used in traditional nonporous films. It would also be beneficial to enhance the mechanical properties of the membrane. This would enable thinner membranes to be utilized. In addition, the operational and maintenance costs of separation systems could he decreased if membranes were not required to be stored in special solutions.